Cavity resonator



Patented Apr. 15, 1952 UNITED STATES PATENT ()FFI'CE CAVITY RESONATORApplication May 12, 1945, Serial No. 593,508

4 Claims.

This invention relates to selective electrical devices and moreparticularly to cavity resonators.

An object of the invention is to provide a cavity resonator which willhave a high Q.

Another object of the invention is to provide a cavity resonator whichmay respond to oscillations of a desired transverse electrical mode andmay discriminate strongly against oscillations of the same frequency ofa transverse magnetic mode.

Another object of the invention is to provide a tunable cavity resonatorwhich may oscillate at a single mode only within its tuning band at eachsetting of the tuner mechanism.

A further object of the invention is to provide a high Q cavityresonator which shall have a reduced mass.

A still further object of the invention is to provide a cavity resonatorhaving a motor-driven tuning mechanism in which the load upon the motormay be very small.

In accordance with the invention a hollow right circular cylinder ofelectrically conducting material capable of excitation to support aninternal electromagnetic field of TEOml'l mode is provided with a diskpiston which may be driven cyclically over the range of the tuning bandby a connected motor. The area of the piston is made smaller than theinterior of the cylinder by an amount of the order of /8 inch radius soas to present at its periphery a high impedance to the radially directedelectric vector of the unwanted transverse magnetic mode (TM!)oscillations having the same frequency as the desired transverseelectric (TEo) mode oscillations. To increase the impedance to undesiredmodes the piston may be provided with apertures located at regions ofWeak field for the oscillations of the desired TEt mode. In one speciesof the invention the piston may be provided with a flange to constitutea guide of high impedance for oscillations of the desired mode while, atthe same time, permitting energy of undesired modes to be dissipated. Inanother modification the edge of the tuning piston may be bevelled toreduce the electrical capacitance in the path of the radial electricvector of undesired modes. To enable the midband frequency to be variedwithout adjustment of the tuning mechanism a separate movable disk maybe used at the end of the resonatoropposite the variable tuner.

In the drawing Fig. 1 illustrates partly in section a cavity resonatorembodying the features of the invention;

Fig. 2 is a sectional view of the structure of Fig. 1 along the plane2-2 viewed in the direction of the arrows;

Fig. 3 shows a modification of the structure shown in Fig. 2;

Fig. 4 is a sectional view of a modification of the tuning piston ofFig. 1; and

Fig. 5 is a sectional view of a further modification of the tuningpiston of Fig. 1.

Referring to Fig. l a cylindrical chamber I! which may be of spun orcast aluminum with an interior plating H! of silver is apertured througha central boss l2 projecting from its lower end to provide a guidewayfor the stem l3 of an adjustably positioned disc 14 silver-plated on itsupper face which serves as the lower boundary of the electromagneticfield Within the chamber. The stem 53 may be moved longitudinally andheld fixed in any position to which it is moved by any well-knownposition adjusting mechanism. At its upper end chamber H is providedwith a cover l5 which may be attached to the chamber by screws IE orother suitable fastenings. Standards I! and I8 integral with the coverextend upwardly therefrom and support a crossframe [9 which carries amotor 20 having a horizontally rotating shaft and a driving eccentric 21connected thereto. Supported from the driving eccentric is areciprocating mechanism 22 terminating in the tuning piston 23 which mayalso be silver-plated on its interior face as at 3| to constitute theupper boundary of the interior electromagnetic field.

The resonator may be excited to oscillate in a TEomn mode by energysupplied from an input circuit 24 to the coupling loop 25 which extendsinto the chamber in a horizontal plane at which oscillations of thedesired TEOmn mode are of relatively high intensity. An output circuit26 may be similarly coupled to the electromagnetic field by a loop 2'!lying in the same plane as the loop 25 at a position preferably but notnecessarily angularly removed therefrom by degrees. Such coupling loopsare relatively ineffective to induce oscillations of undesired TM] modeand accordingly aid in discriminating against such undesired modes.

Concomitant with excitation of oscillations of TEOmn mode is a tendencyfor oscillations of the same frequency of TMlmn mode to be initiated.The presence of such extraneous TMlmn mode oscillations is indicated bytheory and found in practice to reduce the time during which the desiredTEOmn oscillations continue subsequent to withdrawal of the excitingforce. This efiectiv'ely 3 amounts to a reduction of Q of the resonancechamber H for oscillations of the desired mode. Consequently, it ishighly desirable to inhibit or to suppress oscillations of the undesiredTMlmn mode as well as other extraneous oscillations which may tend tooccur with frequencies within the range over which it is desired to tunethe resonance chamber H. One very effective expedient for inhibiting theundesired oscillations of TM mode takes advantage of the fact thatoscillations of the desired 'IEc mode have no radial electric vectorwhile oscillations of the un desired 'I'Mi mode and of other undesiredmodes do involve radial electric vectors. If, therefore, the tuning disc23 be constructed with a diameter such that it fits very loosely withinthe resonance chamber H, the peripheral gap 29 between the margin of thedisc and the interior surface of the cylindrical wall will interpose anattenuating discontinuity in the path of the radial vector of theoscillations of the undesired mode. Moreover, the capacitance betweenthe boundaries to the gap 29 is a factor in determining the frequency ofoscillations of the 'I'M1 mode and hence tends to shift that frequencyaway from the frequency of the TEOmn mode by an amount which, in somecases, may take the frequency of the undesired oscillations outside thefrequency band through which the resonance chamber is to be tuned foroscillations ofthe desired TEOmn mode. It will be appreciated that thegap 29 lies in a region of a relatively weak field for oscillations ofTEo modes.

The discontinuity interposed by the gap 29 tends to cause the radialelectric vectors of the oscillations of unwanted modes to give rise tocurrents traversing the walls of the chamber or space back of or abovethe tuning piston 23 over paths in shunt to the gap 29. These Walls,that is, the portion of the cylindrical surface above the piston 23 andthe inner surface of the cap H3, in fact. all surfaces back of thepiston 23, may be left unplated thus subjecting the unwantedoscillations to additional attenuation. A similar expedient may beemployed in the lower end space or chamber beneath the adjustablypositioned disc I4.

The attenuation of unwanted modes of oscillation having radial electricvectors may be increased by leaving off the silver-plating at the outerperiphery of the tuner discs, both on the surfaces directly exposed tothe cylindrical Walls and for a very small distance on the inner face ofthe discs adjacent their outer peripheries as indicated at 41 and 48. v

The Width of the gap 29, that is, the radial dimension between thecontiguous faces of the cylindrical Wall and the periphery of disc 23 isfound to have an optimum magnitude such that smaller or larger gapsafford relatively less discrimination between oscillations of thedesired mode and oscillations of extraneous modes. The optimum dimensionof this gap is a function of the wavelength of the oscillations of thedesired mode. For example, in a cylindrical cavity resonator designedfor oscillations of TEm mode and having a Wavelength in free space ofapproximately 9 centimeters it was found that the apparent Q of theresonant chamber increased with increase of the width of the gap up to adimen sion of approximately 0.3 centimeter after which the apparent Qwas reduced with further increase of the gap.

The feature of the peripheral gap has a number of other advantages inthe structure shown in Fig. 1. For one thing it eliminates entirely theproblem of variable contact of the tuner disc with the side walls thusremoving a source of considerable electrical instability and ofmechanical wear. At the same time it enables the dimensions and preciseshape of the tuner disc to be less critical. As has already beenexplained, it increases the apparent Q and the ring time foroscillations of a desired TEOmn mode. It also reduces the weight of thereciprocating parts and the consequent load imposed upon the drivingmotor. The selectivity of the resonator for oscillations of the desiredmode as against unwanted or extraneous oscillations is also increased.

Another feature of the tuning disc 23 of Fig. l is the provision ofapertures extending therethrough in regions of relatively weak field asin dicated at 32 and 33. Apertures 32 and 33 are extremely narrow slitsextending in an annular direction to reduce and attenuate the radialvector of oscillations of unwanted modes and, at the same time, toadditionally decrease the mass of the reciprocating parts. Apertures 32and 33, as illustrated in Fig. 2, are interrupted at points in theirlength to leave uncut supporting ribs 34 and 35. In addition to theirfunction of discrimination against oscillations of unwanted. modes infavor of those of desired modes and of reducing the mass of thereciprocating structure these apertures together with the gap 23 reducestill further the load imposed upon the motor 20 in displacement of theair above the tuning disc 23. The adjustable position end member M maybe provided with apertures 36 and 31 similar to the apertures 32 and 33of the disc 23. Its periphery may be bevelled as at 38 to reduce thecapacitance between the periphery and the adjacent cylindrical WallWhile, at th same time, leaving a peripheral gap corresponding to thegap 29. V

Fig. 3 shows an alternative construction in Which the arcuate apertures32 and 33 are replaced by a series of circular holes id and 4 l In acavity resonator designed for oscillations having a free spacewavelength of approximately 9 centimeters and of TEou mode it was foundthat such circular apertures could preferably have a dimension of theorder of 0.6 centimeter.

,Fig. 4 illustrates in section a tuner disc 42 which differs from thetuner disc 23 in that its pe-.

ripheral edges while designed to provide a gap 43 equal to the gap 29are bevelled as at M to reduce the peripheral capacitance to a minimum.This tuner disc may be provided with apertures 45 and 46 which may besimilar either to the arcuate apertures of the structure of Fig. 2 orthe circular apertures of the structure of Fig. 3. It has been foundpossible to so reduce the peripheral capacitance of the tuner disc thatthe resulting high reactance displaces the frequency of the oscillationsof the undesired TM1 mode entirely outside the band through which theapparatus is designed to tune.

Fig. 5 shows a modification of the piston structure in which arearwardly extending annular flange 49 is provided, the outer marginalsurface of which constitutes with the interior surface of thecylindrical wall an annular wave guide having a depth d measured fromthe front surface of the disc to the rear margin of theflange 49. Thisdepth is preferably made such as to discriminate against leakage of thedesired TEo mode oscillations through the gap 29.

It will be understood that in operation the motor 20 will actuate thetuning disc 23 to cause the natural frequency of oscillations of thedesired mode to vary through a band of frequencies extending from alower limit to an upper limit, the width of the band being determined bythe extent of the motion of the disc 23. The midfrequency of the bandand its limiting frequencies as well may all be increased or alldecreased by an appropriate adjustment of the position of lower end disc14. The resonator l I will undergo variation of its natural frequency ata cyclic rate corresponding to the speed of the horizontal rotatingshaft of the motor 29. Incoming energy impressed upon the resonator H bythe input circuit 24 will excite the resonator at its natural resonancefrequency when the position of the moving piston 23 is such that thenatural resonance frequency is in agreement with the frequency of theincoming energy. At such instants the resonator II will serve to supplyenergy of its natural resonance frequency to the output circuit 26 toenergize an indicator or other load element which may be connectedthereto. The resonator ll therefore serves as an electrical oscillationselector of high Q coupling the input circuit 24 to the output circuit26. If it be desired to utilize the resonator H as a so-called echo-box,that is, a resonator in which the microwave energy of brief pulses maybe stored up for retransmission upon cessation of the pulse, the outputcircuit 26 may be omitted. Under such circumstances microwave energy ofbrief pulses incoming over the circuit 24 will excite a strong fieldwithin the resonator ll when the resonator is tuned to the frequency ofthe incoming microwaves and upon cessation of the incoming pulse willfeed back to the circuit 24 oscillations of the same frequency having adecrement depending upon the Q of the resonance chamber ll.

What is claimed is:

1. A cavity resonator comprising a cylindrical chamber having aninterior surface of electrically highly conductive material, a movableend wall positioned in said chamber with a uniform spacing between itsperiphery and the adjacent side walls of the chamber, said peripherybeing so tapered as it approaches its margin as to greatly reduce theelectrical capacitance between the periphery of the piston and theadjacent side walls said gap providing a capacitance sufficient to shiftthe frequency of undesired TM modes outside of the operating range.

2. A cavity resonator having a movable tuning piston serving as one Walland separated at its periphery from the remaining walls by a uniformclearance, the peripheral margin of said piston being bevelled to soreduce the capacitance to the adjacent walls as to introduce arelatively high impedance for oscillations tending to pass from thesurface of said piston to the contiguous surfaces of the other wallssaid gap providing a capacitance suflicient to shift the frequency ofundesired TM modes outside of the operating range.

3. The structure of claim 1, said end wall being provided with arcuateslits for suppressing extraneous modes having a radial electric fieldvector.

4. The structure of claim 2, and extraneous mode suppression devices onsaid piston located at positions of relative weak field intensity forTEOn-m modes.

IRA G. WILSON.

REFERENCES CITED The following references are of record in the file ofthis patent:

UNITED STATES PATENTS Number Name Date 2,088,749 King Aug. 3, 19372,151,118 King Mar. 21, 1939 2,197,122 Bowen Apr. 16, 1940 2,203,806Wolf June 11, 1940 2,253,589 Southworth Aug. 26, 1941 2,323,201 CarterJune 29, 1943 2,405,277 Thompson Aug. 6, 1946 2,439,388 Hansen Apr. 13,1948 2,465,639 Edson Mar. 29, 1949 2,471,419 Edson et al. May 31, 1949

